The field of the invention is that of data compression systems and, more specifically, that of structured transmission systems wherein the communications between two terminals are achieved by exchanges between protocol data units or PDUs.
The invention can be applied notably to packet transmission systems (systems implementing a layer 3 procedure according to the seven-layer OSI standardization model), but also to frame transmission systems (layer 2) or systems using transport protocol data units (layer 4). More generally, the invention can be applied to any type of transmission system, whatever the layer (according to the terminology of the OSI standards) of the communications protocol associated with the system considered.
One particular field of application of the invention is that of data exchange systems working according to the X25 protocol standardized by the CCITT. On French territory, the invention can be applied therefore to communications made through the public TRANSPAC (registered mark) network.
The invention relates more particularly to the optimization of communications, i.e. notably the reduction of costs through the reduction of volumes of data transmitted and/or of communication times.
It is indeed known that, especially when a public communications network needs to be used, the costs generally depend inter alia on the number of protocol data units (or subsets of these units) transmitted and on the communication times.
A known way of reducing the volume of the data transmitted is to compress the data to be transmitted.
Many data compression techniques are indeed known, notably for point-to-point links. Thus for example an adaptive compression algorithm called the Ziv-Lempel algorithm has been chosen in the CCITT recommendation V42a for asynchronous modems.
However, in the case of systems working by exchanges of protocol data units, for example according to the x25 standard, several difficulties are generally encountered.
Thus, there is a known system, called DATAMISER (registered mark), that methodically carries out the byte-by-byte compression of the entire flow of data delivered by a terminal. When this flow of data is organized in protocol data units, each comprising zones descriptive of the exchange (header fields) and data zones (data fields), a compression system such as this does not comply with this form of structuring. In other words, it neither recognizes nor keeps the headers. It therefore cannot be used for an exchange going through a packet-switched network.
In order to overcome this drawback, the selective compression of the data zones might be considered. In this case, the headers are preserved integrally, and the quantity of information elements to be transmitted is substantially reduced. In other words, with a source protocol data unit, there is associated a compressed protocol data unit whose header is identical to that of the source protocol data unit.
In practice, it turns out that the efficiency of this technique is not optimal. Indeed, the number of packets (or PDUs) transmitted remains by definition the same. The gain is therefore minimal since partially empty packets are transmitted.
The invention is notably aimed at overcoming these various drawbacks of the prior art.
More specifically, an aim of the invention is to provide a method to optimize the efficiency of communications made on a data exchange network, reducing the number of protocol data units or subsets of units transmitted.
Another aim of the invention is to provide a method such as this that relates to the structure of the protocol data units transmitted. In other words, an aim of the invention is to provide a method such as this that is transparent from the viewpoint of the terminals as well as from that of the transmission network.
More generally, the invention is aimed at providing a method such as this that enables the reduction of at least one of the following parameters:
cost of the call; PA1 duration of the call; PA1 volume of traffic generated by the network (in terms of number of PDUs and/or bytes) without losing any element of the information transmitted. PA1 the reception of said first sequence of source protocol data units, PA1 data compression associating, with each of said protocol data units, a compressed data sub-field comprising data corresponding to the compression of the data of the data field of said source protocol data unit, PA1 the generation of data for the reconstruction of said source protocol units, PA1 the concatenation of said reconstruction data and said compressed data sub-fields in a sequence of concatenated data, PA1 the slicing of said sequence of concatenated data into data blocks having a predetermined length chosen from a sub-set of at least one possible length, PA1 the creation of said second sequence of protocol data units to be transmitted, the data field of each of said protocol data units to be transmitted comprising at least one of said data blocks. PA1 a step for the association (26), with each of said compressed data sub-fields (30), of a sub-header (29) comprising at least one designator designating the position of the end of said sub-field in the data field of the corresponding protocol unit to be transmitted (51.sub.i); PA1 a step to generate a marker having a predetermined content, known to said receiver terminal; PA1 a step for the association, with each protocol data unit to be transmitted (51.sub.i), of a descriptor of the content of the data field of said protocol data unit. PA1 implementing a compression algorithm that associates a compressed data field with a source data field; PA1 comparing the length of said source data field and compressed data field and selecting the shortest data field as the compressed data sub-field. PA1 searching for and analyzing said data enabling the reconstruction of said first sequence of protocol data units, PA1 the decompression of said data corresponding, in compressed form, to the data contained in the data fields of the source protocol data units; PA1 the restitution of said first sequence of source protocol data units.
Another aim of the invention is to provide a method such as this that can be implemented on any structured system of data transmission.
The invention is also aimed at providing a device, implementing a method such as this, that can be inserted between a terminal and a communications network transparently, i.e. without requiring any adaptation either of the terminal or of the network.
These aims as well as others that shall appear hereinafter are achieved, according to the invention, by means of a data compression method of the type applied to a system of data transmission by the exchange of protocol data units between at least one transmitter terminal and at least one receiver terminal, each of said protocol data units comprising a header field and a data field, said method associating, with a first sequence of N source protocol data units, a second sequence of M protocol data units to be transmitted, M being smaller than or equal to N, at least certain of the data fields of said protocol data units to be transmitted comprising data corresponding to the compression, according to at least one compression algorithm, of data contained in the data fields of at least two of said source protocol data units, and at least certain of said protocol units to be transmitted comprising reconstruction data enabling the reconstruction of said first sequence of source protocol data units from said second sequence of protocol data units to be transmitted.
In this way, the efficiency of the call may be greatly increased but the initial structure of the data is preserved. It is indeed possible, at reception, to reconstitute the source sequence by means of the reconstruction data added to the transmitted data.